1. Field of the Invention
The present invention relates to a method for forming an electrochromic film and an electrochromic device using the same. More particularly, it relates to a method for forming an electrochromic film exhibits excellent reversibility after heat-treatment.
2. Description of the Prior Arts
An electrochromic device is prepared by facing a pair of substrate (1) having an amorphous electrochromic film (3) such as tungsten or molybdenum oxide film and a transparent electrode (2) made of tin oxide etc., and sealing them with a sealant (4) and filling an electrolyte (5) containing ions such as Li.sup.+ and H.sup.+ which colors the electrochromic films, as shown in FIG. 1. If necessary, a pigment such as TiO.sub.2 powder or a thin porous separator made of Al.sub.2 O.sub.3 is incorporated in the electrolyte.
The electrolytes used in the electrochromic device highly affects to reliability of the electrochromic device. Various electrolytes have been proposed. In view of long life of the electrochromic device, it has been well known to use an electrolyte prepared by dissolving LiClO.sub.4 in an organic solvent such as propylenecarbonate, and acetonitrile at a concentration of 1 mole/liter in U.S. Pat. Nos. 3,521,941; 3,708,220; 3,978,007 and 4,059,341.
The sealant for sealing such electrolyte is significantly important to highly affect to the reliability of the electrochromic device as well as the electrolyte. The required characteristics for the sealant include high bonding strength, low moisture permeability, chemical resistance to the electrolyte used and low sealing temperature. The sealing temperature is dependent upon the characteristics of the electrochromic film. The amorphous WO.sub.3 may lose its significant electrochromic characteristic by a heat-treatment at a temperature higher than 300.degree. C. even though it has been used.
Heretofore, amorphous WO.sub.3, and MoO.sub.3 have been used as the electrochromic film. Recently, V.sub.2 O.sub.5 have been known as the electrochromic film. Among them, the WO.sub.3 film has been widely studied and its electrochromic characteristic has been studied in detail.
It has been known to prepare a WO.sub.3 film by a spraying method, a vacuum evaporation method and a sputtering method. Because of low heat resistance of WO.sub.3, the electrochromic characteristic is lost when the temperature of the substrate in the formation of film is too high such as higher than about 320.degree. C. in the spraying method and higher than about 300.degree. C. in the vacuum evaporation method.
In the heat treatment of the resulting electrofilm, it has been known to substantially lose the electrochromic characteristic by heating it at about 300.degree. C. (H. R. Zeller and H. U. Beyeler, Electrochromism and Local Order in Amorphous WO.sub.3 ; Applied Physics Vol. 13, page 231-237 (1977)).
In the sputtering method, the characteristic of the resulting film is not stable. The authors have reported various films such as a film having no electrochromic characteristic and a film being similar to the film obtained by the vacuum evaporation method. These WO.sub.3 films concern to its heat resistance or non-stability.
Accordingly, a glass frit having high reliability which has been used in a liquid crystal device can not be used when the amorphous WO.sub.3 is used.
On the other hand, a sealant having lower sealing temperature range such as an epoxy sealand has not been enough to give reliability after sealing the electrolyte. Accordingly, the inventors have proposed an electrochromic device which is sealed with a sealant made of an ethylene-tetrafluoroethylene copolymer (Japanese Patent Application 6721/1978; filing date of Jan. 26, 1978). The temperature in the heat-treatment is ranging from 250.degree. C. to 350.degree. C. which causes to lose the electrochromic characteristic of the conventional amorphous WO.sub.3.